Usually, a radial type rotor is mounted fixedly on an output shaft by attaching end plates to the opposite ends of the rotor, and fixing the end plates to the output shaft. A shrinkage fit is another method of fixedly mounting a rotor on an output shaft. Another method is the use of pins for fastening a rotor to an output shaft. Most rotors are impregnated with an impregnant to prevent the play of the component rotor cores and magnets thereof relative to each other.
When the rotor is assembled by either the method employing a shrinkage fit or the method using pins, a gap is formed between the rotor and the output shaft, except for end portions of the rotor, and therefore, the output shaft within the rotor is bent when an external force acts on the rotor assembly. The method employing a shrinkage fit is unable to fasten a long, high-torque rotor to an output shaft with a high reliability, because torque is transmitted from the rotor to the output shaft through only the two front and rear portions where the rotor and the output shaft are joined together by a shrinkage fit, regardless of the length of the rotor, while the output torque of a longer rotor is greater than that of a shorter rotor.